1. Field of the Invention
This invention relates to an apparatus for fabricating plasma-generated coatings for medical devices such as stents. The invention also relates to method of coating such devices.
2. Background
Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Stents are usually compressible, so that they can be inserted through small cavities via catheters, and then expanded to a larger diameter once they are at the desired location. FIG. 1 illustrates one example of a conventional vascular stent 10. The stent 10 includes struts 12 connected by elements 14. The combination of the struts 12 and the elements 14 define a tubular body of the stent 12. The tubular body has an outer surface 16 and an inner surface 18. Although the rate of restenosis has been reduced by mechanical intervention of stents, restenosis still presents a significant medical problem. Accordingly, stents have been modified to function not only as mechanical scaffolding, but also to provide biological therapy.
Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
One conventional method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
One of the drawbacks and disadvantages associated with the use of medicated stents has been the aggregation of platelets on the device. A high degree of such aggregation, combined with the early onset of monocyte activation, is believed to be a factor leading to restenosis. One way to reduce platelet aggregation, as well as the early onset of monocyte activation, is believed to be by forming the stent coating using the process of plasma polymerization. Plasma polymerization, also known as glow discharge polymerization, is a method of polymerizing organic substances from vapor phase at low pressures. Plasma polymerization is generally performed by introducing a gas including one or more monomers into a vacuum zone in which the substrate to be coated is placed. The polymerizable monomers are then subjected to an electric discharge to generate ions and/or free radicals. While plasma polymerization can produce coatings having good properties, improvements in the quality of the coatings is desired. For instance, due to the harsh conditions existing in the plasma environment, some areas of the plasma-formed stent coatings can include defects such as burn marks, excessive roughness, and sometimes even delamination. It is desirable to eliminate or at least minimize these problems. Additionally, existing plasma polymerization technologies allow coating of only one stent at a time. To increase production of drug eluting stents, it is desirable to be able to form a plasma polymerized coating on many stents simultaneously. The embodiments of the present invention address these and other issues associated with coating of implantable medical devices.